Sulfide stress cracking is a form of metal failure which is different from, and not necessarily related to normal corrosion, and which may occur even in the absence of substantial oxidation, hydrogen generation, or metal removal. Sulfide stress cracking normally occurs in metals under an applied or a residual stress resulting, for instance, from cold working. Sulfide stress cracking moreover can occur even though a metal is inhibited against normal corrosion. Thus, typical corrosion inhibitors are relatively ineffective in the prevention of sulfide stress cracking.
Sulfide stress cracking manifests itself by cracks or fissures that appear on the surface of stressed metal upon its exposure to a hydrogen sulfide-containing environment. After the cracks occur, the metal exhibits greatly reduced strength in the area of the crack and tends to break apart upon continued application of stress. Although sulfide stress cracking is not fully understood, it is thought that free hydrogen atoms enter the stressed metal and lodge between the metals grain boundaries and then combine to form molecular hydrogen, thereby exerting a great force within the surface of the metal. From these considerations flow the conclusions that sulfide stress cracking is a process different from ordinary ferrous metal corrosion: metals that are effectively protected from corrosion by known corrosion inhibitors are still vulnerable to hydrogen sulfide stress cracking, the corrosion inhibitors providing no apparent resistance to the cracking phenomenon.